genera, viz., Unio and Anodonta. This is the distinction between 
them. In Unio the hinge, or point upon which the valves work, 
is supplied with projections known as teeth, and corresponding 
receptacles. In Anodonta the hinge-line is quite smooth and 
toothless. There are three kinds of Unio found in England— 
firstly the famous pearl mussel, which is confined to mountain 
streams, and therefore does not occur at York; secondly, Unio 
jnctoriim, so-called because painters used the shell for palettes; 
and thirdly, Unio tumidus. The last two occur in both our rivers. 
In the genus Anodonta^ Jeffreys makes two species, Anatina, in 
which the hinge line is carried into a sort of crest, and Cygncea, 
which has the hinge line almost parallel with the other edge of 
the shell. This distinction, however, is difficult to preserve ; one 
form passes by imperceptible grades into another. Both forms 
occur abundantly at York. *The mussels can be collected in 
plenty only when the locks are open. The water then sinks 
sufficiently low to disclose large numbers of them sticking in the 
soft banks. When Nabmm Lock is opened that small portion of 
the River Foss (only some 200 yards in length) wLich lies 
between Castle Mills Lock and the Blue Bridge is almost 
drained. Yery near the mouth of the Foss occurs a number of 
mussels belonging to the species Anodonta anatina. They are 
remarkable for possessing a beautiful ornamentation of rich 
green rays. The epidermis is lustrous, the interior highly 
nacreous. Now, by walking a few yards we find ourselves 
following the bank of the Ouse. Here occurs the very same 
form of Anodonta, but how changed in appearance ! Instead of 
a lustrous green epidermis the shell is of a dark dead brown 
colour, the pearliness of the interior is quite dull, and the 
phenomenon of erosion, or the eating away of the epidermis and 
upper layers of the shell, is extensively developed. Erosion is 
caused by the presence of carbonic acid or by the rapiditj^ of 
the current of water. Probably the former has caused the 
disfigurement of the Ouse Anodontas, owing to the drainage 
matter present in the river just below York. Exactly the 
same contrast is shown in the specimens of Unio pictorum, from 
the same two localities, with the additional distinction that the 
Ouse specimens have a slightly curved form, and belong to the 
